How do I get the Color value from an RGB node with Python?
Welcome to Blender.SE, UlfR.

Finding out a property in Blender is relatively easy. Make sure, you have python tooltips enabled (in the user preferences).

Then hover your mouse cursor over the property (in this case the color). A tooltip containing the python command will appear.



The property you seek seems to be outputs[0].default_value.

def_rgb = bpy.data.materials['MyMaterial'].node_tree.nodes['RGB'].outputs[0].default_value


It is a bpy_prop_array by default, so you may want to convert it to a list.

float_values = list(def_rgb)